1. Field of the Invention
The present invention relates to additives which comprise a cellulose ether composition for use as water-retention agent, plasticizer and lubricant in the extrusion of mineral masses. In addition it relates to the use of this cellulose ether composition in building material systems and also to a process for the extrusion of mineral masses using these additives. The use of this methylcellulose composition leads, in the extrusion process, to improved process properties and to a higher surface quality of the extruded body.
2. Brief Description of the Prior Art
The extrusion of mineral masses has been employed industrially for many years. Of particular interest here is the extrusion of cement-bonded masses.
Cement extrusion is a process for shaping pasty cementitious mixtures into any desired profiles by pressing them through a die mouth. The elements obtained in this manner may be used in many ways, in particular in construction applications. Here they can firstly replace cement masses which have been cast hitherto, but secondly, they make accessible profile shapes which are not accessible by the conventional process of casting. Examples of extruded building elements range from small angle profiles which have a width of only a few centimeters up to large building slabs which can have a width of 60 cm and theoretically any length. Extruded cement masses comprise, as constituents, in principle cement as binder, possibly other binders, aggregates (sands) and/or lightweight aggregates, and cellulose ethers, in particular methylcellulose, as water-retention agent, plasticizer and lubricant.
Methylcellulose in this publication is taken to mean all methyl-group-containing cellulose ethers such as methylcellulose, methyl hydroxyethylcellulose, methyl hydroxypropylcellulose, methyl hydroxyethyl hydroxypropylcellulose, methyl ethylhydroxyethylcellulose and methyl hydroxyethyl hydroxybutylcellulose. Frequently, natural and/or synthetic fibres are also added to the formula. The amount of the methylcellulose or the cellulose ether composition used is generally between 0.5 and 6%, based on the extrusion mass without water.
Whether a mass may be extruded and may be processed to give marketable products is determined by various parameters. The mass must have highly plastic behaviour during the extrusion process and should be able to be discharged with the lowest possible pressure and the highest possible rate homogeneously from the die mouth. Here, the surface of the extruded mass plays a critical role. It should be crack-free and as smooth as possible. Good surface properties are a problem especially after a relatively long extrusion time, since after a relatively long operating period higher temperatures are established through the friction of abrasive mineral particles. The quality of the surface of an extruded mass generally decreases more with higher the temperature of the extruded mass. Extrusion temperatures around 60° C. are considered to be particularly critical. All these problems have been solved only inadequately hitherto.
Numerous patent applications claim additive compositions which are said to beneficially affect various parameters of the extrusion process or properties of the end product. WO 01/16048 A1 claims a mixture of viscosity enhancing agents (including cellulose ethers) and dispersion agents where synergistic effects between the two components reportedly result in a reduction in dosage. As dispersion agent, various sulphonates are claimed. Polymers based on acrylic acid, acrylic esters and other acrylic polymers are mentioned but not specified in more details. These polymers such as polyacrylates, because of their lack of capability for a strong three-dimensional crosslinking, are completely water-soluble and are markedly of lower-molecular-weight than superabsorbers based on polyacrylate. While they can be slightly crosslinked in a linear or branched manner, they always remain completely water-soluble. In a cementitious mortar system such as the abovementioned system of the extruded cement masses, they lead to an unwanted decrease in water requirement. In contrast the use of superabsorbers significantly increases the water requirement. It was therefore not to be expected that superabsorbers, despite their similar chemical composition, improve the extrusion properties of a cement mortar at high temperatures.
EP-A-0 131 090 A1 describes mixtures of water-soluble polysaccharide polymers based on cellulose, guar or starch with a water-swellable but not water-soluble polymer based on a partially hydrolyzed polyacrylamide for use in various building materials (such as cement-bonded tile adhesives and plasters, gypsum-bonded plasters, wallpaper adhesives and concrete). However, only cement-bonded tile adhesives are considered in more detail. Compared with a mixture of a polysaccharide polymer and an uncrosslinked (and water-soluble) polyacrylamide, the mixtures with crosslinked partially hydrolyzed polyacrylamides exhibit a better open time and generally higher tensile bond strengths of the tile adhesive under test. Even for those skilled in the art, it is not at all obvious herefrom that a similar mixture of cellulose ether and superabsorbers improves the surface properties and other properties described in the case of the extruded cementitious mass at relatively high temperatures. The processing temperature of the building materials mentioned in the above-cited EP-A-0 131 090 A1 is generally in the range of room temperature. The high extrusion temperatures do not occur in the case of other extruded masses, for example in the case of clay, or the ceramic cited in EP-A-0 131 090 A1, since in both these cases the friction of the particles against one another is less and thus the heating of the mass is markedly lower. In particular sheet silicates, as are present in clay, have a good lubricating action.
EP-A-327 351 A2 claims a blend of methylcellulose and crosslinked insoluble polyacrylic acid for improving the sag resistance in thin-bed mortars, for example tile adhesives. Here also, it would not be possible for a person skilled in the art to infer surface effects in extruded cement masses either from the effect of improving the sag resistance or from the customary surface temperature of thin-bed mortars.
JP 10-152357 claims extrudable mortars which comprise a redispersible and uncrosslinked synthetic polymer which has been produced by (normal or inverse) emulsion polymerization with the aid of a polymeric protective colloid and a surfactant and has then been dried by a suitable process to give a powder. By adding these redispersible polymers to the mortar mixture, higher bending tensile strengths and lower extrusion pressures are obtained. These products are produced in a different manner from the above-described superabsorbers and, in contrast to superabsorbers, are not crosslinked, even if they can contain acrylate groups. Extrusion properties at elevated temperatures are not described. There is likewise no report of a higher surface quality.
JP 4-164604 claims the use of strongly water-absorbent polymers as additive to extrudable cement masses in dosages of 0.1-2% by weight. In this case, the water-absorbent polymers are used in addition to the methylcellulose used to obtain, via the greatly increased water requirement, lightweight building materials since, after the cement has set, and the mass is dried, the previously swollen polymers leave behind pores. There is no report on extrusion properties at elevated temperatures. Disadvantages, as a result of the high water requirement, are in particular the falling bending strength and compressive strength of materials produced in this manner.
The object therefore underlying this invention is to find a water-retention agent, plasticizer and lubricant which have improved properties with respect to the extrusion process and the surface quality of the extruded profile, in particular at elevated temperatures. Elevated temperatures here are taken to mean temperatures of 30-65° C., and in particular 45-65° C. These improved properties can mean, for example, a lower discharge pressure at the die mouth, a smoother surface of the extrudate, lower crack formation of the extruded profile or a higher extrusion rate.